1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to electrostatic chucks for holding wafers during processing.
2. Prior Art
Electrostatic chucks (ESCs) for use in semiconductor processing tools are well-known in the prior art. A major advantage of such ESCs is the manner in which wafers are held on the chuck. By utilizing DC biased electrodes, electrostatic charges of opposite polarity form in the wafer. The electrostatic force maintains sufficient attraction to retain the wafer proximal to the upper surface of the chuck without the use of mechanical retaining means.
Prior art ESCs are designed so that the main housing of the chuck is of one polarity while an electrode of opposite polarity is made to reside in or on the chuck proximal to the upper surface. A wafer is then placed above the chuck for processing. Although performance is adequate in most instances, two problems are noted with the prior art bi-polar ESCs.
Firstly, since the housing of the chuck functions as an electrode, the impedance across the surface of the chuck is not uniform. This is mainly due to the difference in radio frequency paths to the wafer through both electrodes from the RF source. Secondly, due to the attraction of electrons (or ions) to a polarized electrode, plasma leakage around the edge of the wafer is noticeable, especially when this leakage contributes to the breakdown of the dielectric coating on the chuck. Plasma shaping rings have been utilized to shape the plasma sheath to obtain a more uniform plasma, but have not been successful in preventing this plasma leakage.
The present invention describes an electrostatic chuck that attempts to remove or reduce these shortcomings.